Processes for the co-polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted, for example, by introducing the gaseous monomer and comonomer into a stirred and/or gas fluidised bed or sub-fluidised bed comprising polyolefin and a catalyst for polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
In the polymerisation of olefins in a sub-fluidised bed reactor, the bed of polymer particles is generally maintained in a sub-fluidised state by means of an ascending gas stream and mechanical agitation. An example of such a process is given in WO 99/29406.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst would become inactive or the bed would begin to melt. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying the fluidising gas to the polymerisation reactor at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas is used to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
It is also well known that chromium oxide or “Phillips” catalysts can advantageously be used for the (co-) polymerisation of olefins, particularly in slurry processes as well as in gas phase processes. It is even reported in the literature that chromium oxide catalysts are much less or even not prone to fouling problems in olefin gas phase polymerisation processes in comparison with Ziegler-Natta catalysts which exhibit many fouling problems.
Process aid additives are known to be added to processes for the (co-) polymerisation of olefins in order to try and mitigate problems with fouling.
WO 01/18066 and WO 01/18067, for example, describe the use of a process aid additive in the (co-) polymerisation of olefins in a fluidised bed reactor using a chromium oxide catalyst and a Ziegler-Natta catalyst respectively. WO 01/18066 and WO 01/18067 both disclose, generally, that the additive may be added at any location of the fluidised bed polymerisation system. When a cyclone is present in the recycle line (to separate entrained fine particles from the recycle gas), however, the preferred position for addition of the additive is in the fines recycle line, i.e. after the cyclone, or directly into the polymerisation zone.